System for transmitting high-speed added-value services in terrestrial digital broadcasting

ABSTRACT

A broadband multi-frequency-block Extended Digital Audio Broadcasting X-DAB) transmission method for the interference-free transmission of value-added services at a high data rate to mobile receivers within an analog television channel. At least two simultaneously-broadcast adjacent X-DAB frequency blocks in a single-frequency network are provided. A digital input data stream is multiplexed onto the frequency blocks, with the input data steam being source-coded according to the service content of the input data stream. The input data stream is separated into a plurality of individual dens streams, with the individual data steams each having a respective quality of service importance and being distributed according to the respective quality of service importance among the at least two frequency blocks. The method offers a high degree of flexibility with regard to error protection profiles and thus data rates, and permits the implementation of non-constant-ratio error protection profiles, as well as hierarchical transmission.

FIELD OF THE INVENTION

The present invention relates generally to a method for the transmissionof high-rate value added services, and, in particular, to a method forthe transmission of high-rate value-added services according to theExtended Digital Audio Broadcasting (X-DAB) system.

RELATED TECHNOLOGY

The present invention lies in the field of the future digital radiobroadcasting: “Digital Audio Broadcasting” (DAB), which was standardizedin February 1995 by the European Telecommunication Standards Institute.DAB is suitable for the transmission of qualitatively high-grade audioprograms to mobile, portable and fixed receivers, the real objectivebeing mobile reception. One of the special features of DAB is thetransmissibility of additional data at normally relatively low datarates, such as program-accompanying information, traffic information andthe like. A plurality of audio programs and data services, respectively,are combined in a “DAB ensemble” and are broadcast jointly by the COFDM(Coded Orthogonal Frequency Division Multiplexing) multi-carriertransmission process in a frequency block of a width of approximately1.5 MHz.

Recently, program providers have shown great interest in using DAB alsoto transmit value-added services such as video programs at a higher datarate. In the limiting case, this data rate may occupy the capacity ofthe entire DAB ensemble. One of the problems in this regard is that oferror protection, which, at the maximum net data rate of 1.728 Mbit/susable in the DAB system is so weak that the system is not suitablewithout restriction for mobile reception. Therefore, with acceptableerror protection, interference-free transmission is only possible in therange of data rates up to approximately 1.2 Mbit/s.

Relative to the DAB system, the X-DAB system (Extended DAB) represents adownward-compatible, qualitatively higher-grade alternative to the DABsystem, the X-DAB system retaining unchanged the physical parameters ofthe utilized OFDM multi-carrier transmission process as well as thebasic structure of a transmission frame according to the DAB system. Forspecial X-DAB data channels, in comparison with the DAB system, it ispossible to obtain better interference immunity, accompanied at the sametime by a higher data rate, through the use of coded modulation(multi-stage codes in conjunction with a higher-stage phase modulationof, for example, 8-PSK). Implementation of the X-DAB system makes itpossible at any time to transmit a net data rate of 1.728 Mbit/s withoutrestriction to the mobile user per DAB frequency block. It is evenpossible to achieve a maximum rate, of over 2 Mbit/s, but with anincrease in the required signal-to-noise ratio at the receiver end incomparison with the lower data rate.

If the intention is to transmit high-grade value-added services, such asvideo programs in PAL quality, to mobile users, then the data ratesachievable by the DAB and X-DAB systems are still too low. At present, astandard for the terrestrial broadcasting of digital television (DVB-T)is soon to be published by the responsible authority, the ETSI. It maybe that the DVB-T system makes it possible to transmit high datarates—namely up to a maximum of about 30 Mbit/s per television channelof a bandwidth of 7 MHz in the VBT range or 8 MHz in the UHF range;however, the system concept, which is likewise based on the COFDMmethod, is designed primarily for supplying stationary and portablereceivers. That is, mobile reception is possible only to a very limitedextent even if use is made of a non-standardized expansion of the DVB-Tsystem with regard to the type of modulation. See, for example, GermanPatent No. 4319 217 C2, which is hereby incorporated by referenceherein. In order just to come approximately within the range of thereceiver-end signal-to-noise ratio available with the DAB system, it ispossible with the DVB-T system to employ no more than a four-phasemodulation on the subcarriers, as is used also in the DAB system. Thatis, in this case, the maximum transmissible data rate per televisionchannel still amounts to approximately 6 Mbit/s.

In conjunction with the DAB system, German Patent Application No. 43 06590 A1, which is hereby incorporated by reference herein, has proposedthat four adjacent frequency blocks be broadcast simultaneously in onetelevision channel, the correspondingly source-coded data stream (suchas MPEG-2 for video and audio) being multiplexed onto the frequencyblocks, whereby with a broadband DAB receiver, composed of fourparallel-arranged receivers arranged with a final output combiner, it ispossible to evaluate a net data rate of max. 4×1.728=6.9 Mbit/s. If anerror protection adapted to mobile conditions is used, this net datarate will still be approximately 4.6 Mbit/s. However, there is a need tofurther increase this data rate in order to optimize the transmissionquality.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for thetransmission of high-rate value-added services which reliably ensurestransmission at even higher data rates than previously possible with theDAB system.

The present invention provides a method for the transmission of videoprograms, particularly to mobile users, utilizing at least twosimultaneously broadcast, adjacent X-DAB frequency blocks in a singlefrequency network, a digital input stream, source-coded according to therespective service contents, being multiplexed onto the frequencyblocks. According to the invention, a free carrier frequency is insertedbetween the X-DAB frequency blocks. It is thus possible to dispense witha guard separation of 0.2 MHz or greater between two frequency blocks.In this manner, the band-width requirement for the X-DAB frequencyblocks is reduced, so that it is advantageously possible, within achannel having a predefined band width, to transmit a higher number ofadjacent X-DAB frequency blocks within the predefined band width, thuspermitting a higher transmission rate.

According to an embodiment of the present invention, in order toincrease the transmission rate, according to the invention, the inputdata stream is separated into a plurality of individual data streams,these individual data streams having different significance orimportance for the quality of the service, and the individual datastreams are distributed, according to their significance or importance,between individual X-DAB frequency blocks. This transmission of a datastream, also described as hierarchical transmission, makes it possibleto strengthen error protection for more important partial data streamsor individual data streams, and to reduce the error protection for thosewhich are less important, but in exchange, to transmit a higher datarate in the case of the partial data streams which are less important.This makes it possible to increase the transmission data rate overall.

Advantageously, during transmission according to the invention, theoutput signals of the four X-DAB narrow-band transmitters, which providethe four X-DAB frequency blocks, are combined into one common outputsignal and are thereupon transmitted, for which purpose a summingcircuit is provided whose output is connected to an HF stage. At theinput end, during the transmission process, the input data stream issplit at the input end between the four X-DAB transmitters by aservice-splitting circuit.

As an alternative to the summing of the output signals from theplurality of X-DAB transmitters, in order to prevent disturbances of thecondition of orthogonality between the subcarriers of the adjacentfrequency blocks, provision is advantageously made for combining thesignals corresponding to the plurality of X-DAB transmitters at thelevel of the digital baseband signal processing after a differentialmodulation of the subcarriers of individual blocks, yet before theactual OFDM signal generation. Advantageously, the OFDM signals areexpediently generated by Inverse Fast Fourier Transformation (IFFT),followed by D/A conversion and I/Q modulation.

The bundling of a plurality of, for example four, X-DAB frequency blocksto the advantage of a broadband X-DAB, BX-DAB system within a normaltelevision channel permits the interference-free transmission ofvalue-added services at a high data rate in particular to mobilereceivers at a typical data rate of 6.9 Mbit/s.

The system concept according to the present invention offers a highdegree of flexibility with regard to different error-protection profilesand thus data rates, the profiles, in the transmission of a data frame,being switchable within a frequency block and also between the blockswith regard to the individual data streams of the source signal. Incontrast to the above-described DVB-T system, this makes it possible toimplement both a constant-ratio and a non-constant-ratio errorprotection. The system concept is also suitable for a type ofhierarchical transmission, where, in contrast to the DVB-T system, thedifference lies not in the type of modulation on the individual OFDMsubcarriers, but in the number of frequency blocks to be evaluated.

BRIEF DESCRIPTION OF THE DRAWINGS

Following, the preset invention is described in greater detail by way ofexample, with reference to the drawings, in which:

FIG. 1 shows the structure of the transmission frame for the X-DABsystem for each frequency block;

FIG. 2 shows a block diagram of a narrow-band X-DAB transmitter;

FIG. 3 shows a block diagram of a narrow-band X-DAB receiver;

FIG. 4 shows the distribution of the X-DAB frequency blocks within ananalog television channel as part of the yielding of a transmissionsignal according to the transmission method of the present invention;

FIG. 5 shows a block diagram of a broadband X-DAB transmitter;

FIG. 6 shows a block diagram of a broadband X-DAB transmitter;

FIG. 7 shows a block diagram of a broadband X-DAB receiver; and

FIG. 8 shows a block diagram of a broadband X-DAB receiver.

DETAILED DESCRIPTION

As noted above, there is a need for new system concepts making itpossible to also provide value-added services at higher data rates formobile receivers without any appreciable loss of quality. Throughrecourse to already existing systems such as DAB or, as in the caseaccording to the invention, to X-DAB as the basic structure, it ispossible to significantly reduce the costs of hardware development forthe transmitter and receiver ends.

For example, four different parameter sets (transmission modes) whichdescribe the physical parameters of the transmission frame and of theOFDM process exist for the DAB system. These parameter sets areidentical for the X-DAB system. For example, in the “transmission modeII”, a frame corresponds to a duration of 24 ms and contains L=76 OFDMsymbols, of which the first is occupied by the synchronization signaland the next 1-3 by the control channel (fast information channel, FIC),as shown in FIG. 1. The remaining symbols, which form the main servicechannel (MSC), are available for the transmission of useful data. Ablock in the transmission frame corresponds to the data contents whichcan be transmitted within an OFDM symbol. Each service occupies its ownarea, the so-called subchannel, in the MSC. Also integrated in the MSCis the XSC (X-DAB Service Channel) which, in extreme cases, may take upthe entire MSC capacity. However, such an extreme case occurs only if aservice is to be transmitted at a high data rate, i.e. in this case, theMSC or XSC contras only one subchannel. The number K of subcarriers usedin the OFDM process is likewise different for the individualtransmission modes and ranges from K=192 for mode III to K=1536 formode 1. FIGS. 2 and 3 show corresponding block diagrams of an X-DABtransmitter and receiver, respectively.

In conformance with the proposed design approach according to theinvention, a plurality of, in the present case up to four narrow-bandX-DAB frequency blocks, which are simultaneously broadcast by alltransmitters of a common-wave or single-frequency network, areaccommodated in a television channel with a bandwidth of 7 or 8 MHz (VHFor UHF range), as shown in FIG. 4. The maximum data rate achievablethereby per channel by bundling the capacity of the individual frequencyblocks, i.e. the ensemble contents is approximately 8.3 Mbit/s in thecase of four frequency blocks. If use is made of error protectionadapted to mobile reception, 6.9 Mbit/s thereof is still left for thetransmission of useful data. That is, per data channel, approximately 4MPEG-2 coded video programs in VHS quality including stereo sound, orone to two programs in PAL quality can be transmitted and received bymobile users.

In addition, the capacity of the four fast information channels (FIC) isavailable for the transmission of additional data (Fast Information DataCharnel, FIDC), each with about 32 kbit/s (mode III: 43 kbit/s).

The frequency blocks are advantageously arranged in such a manner thatthe individual subcarriers of all the blocks lie in the sameradio-frequency pattern Δf. The radio-frequency pattern Δf is defined bythe useful interval duration T_(U) of an OFDM symbol: Δf =1/T_(U). Thesmallest carrier interval is in mode 1: Δf=1 kHz, and the largest inmode III: Δf=8 kHz.

It is advantageously possible to dispense with a greater guardseparation of 0.2 MHz, as is required in the customary allocation offrequencies for the DAB system in order to prevent adjacent-channelinterference between individual frequency blocks. Merely one single,unassigned carrier frequency is preferably additionally inserted betweenthe frequency blocks. Taking account of the fact that the carrier on therespective block mid-frequency is also not assigned, the bandwidthrequirement for the four frequency blocks results as:K′*Δf=(4*(K+1)+3)*Δf=(4*K+7)*Δf.

If use is made of the known DAB or X-DAB parameters, there results avalue of approximately 6.2 MHz for all modes. This system concept isreferred to hereinafter as BX-DAB (Broadband X-DAB).

Basically, after an appropriate service splitting of the input datastream, which may possibly originate from a plurality of sources, thedesired broadband transmission signal can be generated by a multiple, inthe present case a four-fold parallel connection of known X-DABtransmitters according to FIG. 2 and subsequent addition of the outputsignals of these transmitters. This situation is presented in FIG. 5. Aseparate controller monitors the service splitting and generates thenecessary multiplex information (MCI) for the FIC. However, owing toinaccuracies of the respective transmission oscillators with thetransmission mid-frequencies f_(A), f_(B), f_(C) and f_(D), thetransmission signal may in this case already contain disturbances of thecondition of orthogonality between the subcarriers of the adjacentfrequency blocks, which, because of the lack of a guard separation, mayultimately result in inter-carrier interference (ICI) and thus in adeterioration of the transmission quality.

FIG. 6 shows a specific embodiment of the transmitter which does notexhibit these disadvantages. Accordingly, the signals are combined atthe level of the digital baseband signal processing (DSP/BB X-DAB SA-SD)after differential modulation of the subcarriers of the individualblocks, and still before the actual OFDM signal generation. The OFDMsignal generation can be implemented by a module for Inverse FastFourier Transformation (IFFT), followed by a D/A converter and then anI/Q modulator. It is of advantage in this regard that, whereas in thefirst embodiment of the transmitter according to the invention for theindividual frequency blocks, it is necessary in each case to use an IFFTof size N>K+1, with N as a power of two (N=256 for mode III to N=2048for mode I), in order to generate a timing signal belonging to an OFDMsymbol, in the specific embodiment shown in FIG. 6, only one IFF of sizeN′>K′ is necessary (N′=1024 for mode III to N′=8192 for mode I). EachX-DAB block is allocated a fixed assignment—dependent on the selectedmode of the IFFT input vector on which the corresponding, differentiallymodulated PSK symbols are stored. Following the IFFT, all that is thenrequired in the transmitter is a single D/A conversion (one module eachfor the I- and Q-channels) as well as a single I/Q modulator. However,owing to the greater IFFT bandwidth, the D/A conversion must be carriedout at four times the clock-pulse rate, i.e. the duration of thesampling interval after the IFFT is only about 0.122 μs instead of 0.48μs in the first embodiment of the transmitter. The frequency f_(S0) isused as the transmission mid-frequency, as shown in FIG. 4.

Broadband X-DAB receivers shall now be explained with reference to FIGS.7 and 8.

In order, at the receiving end of the transmitting, to be able toevaluate the entire data stream in the broadband transmission signal,two principles are possible as shown in FIGS. 7 and 8. First, if thereare four narrow-band X-DAB transmitters at the transmitter end, it ispossible to employ four narrow-band X-DAB receivers in parallel whichand tuned to the respective frequency-block mid-frequencies f_(A) tof_(D) and which thus evaluate the ensemble contents of the correspondingblock (see FIG. 7). Owing to the fact that the frequency blocks aredirectly adjacent to each other, if narrow-band receivers are used,there is inter-carrier interference (ICI) because the subcarriersbelonging to the adjacent blocks are included in the FFT module of theOFDM demodulator. However, this has virtually no adverse effect on thetransmission quality, since, in the previously discussed transmitterimplementation (see FIG. 6), all the carriers are in the sameradio-frequency pattern, with the result that the condition oforthogonality is not violated. Furthermore, the negative consequences ofthe Doppler effect for mobile reception are only minimally amplified dueto this. The four receiver output data streams are again combined in theservice combiner to form one or more overall data streams. The controlof the just-described broadband receiver with regard to the data streamsis effected in the combining controller on the basis of the evaluationof the information transmitted in the FICs, analogous to the DAB orX-DAB concept.

FIG. 8 shows an alternative to the above-described receiver, analogousto the transmitter shown in FIG. 6. Instead of using four narrow-bandreceivers directly in parallel, it is thus possible for the signal to berecovered using just one OFDM demodulator. This broadband receiver canbe implemented by an I/Q demodulator tuned to the frequency f_(S0), anA/D converter for each of the I- and Q-branches, as well as a subsequentFFT of size N′. The values of the FFT output vector can, aftercorresponding allocation, again be allocated to the digital basebandsignal processing of four parallel X-DAB receivers (DSP/BB X-DAB EA-ED)whose output data stream, as described above, are subsequently combinedto form an overall data stream.

This second embodiment of the receiver according to the invention issuperior to its first embodiment when it is a matter of evaluating theentire data stream. However, with the above-described broadband X-DABtransmitter/receiver concept, it is also possible to accomplish aso-called hierarchical transmission of a data steam. This requires thatthe source data stream can be separated into a plurality of individualstreams, these having different significance or importance for thequality of the associated service, for example with regard to imagequality. Thus, it is possible, for example, to increase the errorprotection for more important data streams and to reduce it for lessimportant ones, but in return, to transmit a higher data rate in thisarea. Changes in error protection even within a transmission frame areeasy to implement with the X-DAB system, but are not possible with theDVB-T system described at the outset. If, at the transmitter end, thedata streams are distributed according to their importance among theindividual frequency blocks beginning, for example, with the lowestfrequency, then reception quality and receiver costs can be exchangedfor each other, a technically simple receiver, for example, evaluatingonly one frequency block such as block 1, which is tuned to themid-frequency f_(A). In this case, the receiver is identical with thenormal X-DAB narrow-band receiver with the maximum FFT size N′=N=2048.In order to improve the service quality, it is necessary for a furtherblock to be added and co-evaluated, for example block 2, tuned to themid-frequency f_(S1). Since an FFT of the maximum size N′=4096 isrequired, the receiver must already be correspondingly broad-band. Inthe case of video transmission, this may signify, for example, a qualitystep from VHS to PAL. In this scheme, frequency blocks 3 and 4 maylikewise be assigned to a separate service and be evaluatedindependently of frequency blocks 1 and 2, the receiver being tuned tomid-frequency f_(S2).

With regard to the above-described frequency blocks, it is also possiblefor an additional substream to be transmitted to frequency block 1 or 2,this contributing to a further improvement in the quality of theservice. In order, for example, to achieve full image quality in thiscase, it is necessary to use the most complex and thus most costlyreceiver, with the maximum FFT size N=8192 given four-foldparallelization of the X-DAB baseband signal processing. The arrangementof an unassigned carrier between the individual frequency blocks has,for all receiver types irrespective of FFT size and theaccordingly-selected mid-frequency, the consequence that the inputsignal is always without a direct component, this guaranteeing optimalmodulation of the A/D converters.

1. A method for transmission of high-rate value-added services to mobileusers, the method comprising: providing at least twosimultaneously-broadcast adjacent extended digital broadcasting systemfrequency blocks in a singe-frequency network; multiplexing a digitalinput data steam unto the at least two frequency blocks, the input datastream being source-coded according to a service content of the inputdata stream; and inserting a free carrier frequency between the at leasttwo frequency blocks.
 2. A method for transmission of high-ratevalue-added services to mobile users, the method comprising: providingat least two simultaneously-broadcast adjacent extended digitalbroadcasting system frequency blocks in a single-frequency network;multiplexing a digital input data stream onto the at least two frequencyblocks, the input data stream being source-coded according to a servicecontent of the input data stream; and separating the input data streaminto a plurality of individual data streams, the plurality of individualdata steams each having a respective quality of service importance andbeing distributed according to the respective quality of serviceimportance among the at least two frequency blocks.
 3. The transmissionmethod as recited in claim 1 further comprising: combining signalscorresponding to the at least two frequency blocks into a common outputsignal; and transmitting the common output signal.
 4. The method fortransmission as recited in claim 2, further comprising: combiningsignals corresponding to the at least two frequency blocks into a commonoutput signal; and transmitting the common output signal.
 5. The methodfor transmission as recited in claim 1, further comprising:differentially modulating respective subcarriers of the at least twofrequency blocks; then combining signals corresponding to the at leasttwo frequency blocks into a common output signal; and then generating atleast one orthogonal frequency division multiplex signal.
 6. The methodfor transmission as recited in claim 2 further comprising:differentially modulating respective subcarriers of the at least twofrequency blocks; then combining signals corresponding to the at leasttwo frequency into a common output signal; and then generating at leastone orthogonal frequency division multiplex signal.
 7. The method fortransmission as recited in claim 5 wherein the generating is performedusing an Inverse Fast Fourier Transformation followed by a D/Aconversion and an I/Q modulation.
 8. The method for transmission asrecited in claim 6, wherein the generating is performed using an InverseFast Fourier Transformation followed by a D/A conversion and an I/Qmodulation.